The same can be said about every minor league in sports.. yet they exist. A league can be successful is structured correctly, but if the entire league's sustainability is structured around NFL subsidies, then it might be a good idea to have those deals in place, before you open up shop.
Most minor league sports are affiliated with a pro sports team. Sure, those players are of a lesser talent and most will never see the pros, but those that are better and will advance, do draw interest from fans. They also play in 3,000 - 7,000 seat parks and not 65,000 seat monstrosities like the AAF
is was.
Even college football, though it's big on it's own, has that draw. There's interest in the top players who are going to the next level. The AAF had none of that going. A bunch of guys who aren't very good with no affiliation with the big boys for a chance to move up. It was just a flawed plan from the get go in just that aspect of it.
I think the XFL will play out the same way. If they don't have the talent, and/or there's no chance for players to advance, I don't see them drawing much interest. They'll likely last a little longer because they have more money and better marketing, but people won't watch lesser talent that has nowhere to go.
If the NFL got behind a developmental league, you wouldn't need it to draw 65,000 at games. You could put it in smaller cities without pro teams, play it in soccer or high school stadiums, and it would eventually gain a local following with the staying power of the NFL behind it, just like all other minor leagues do.
I came up with an 8 team concept several years ago where each NFL division would have a minor league team represent it made up from the 54-90 guys who get cut every Fall. Each team in the AFC South, for example, keeps club control of 15 players from those 54-90 guys they cut. So the Springfield, Missouri team would have a roster of 60 players, 15 from each of the 4 AFC South teams. The same for Birmingham and the NFC South, and so on.
That's where this needs to go. Just having another football league for the sake of having one isn't going to work. No one's going around trying to start up another professional baseball league, hockey, basketball. It won't work unless you take top talent like the AFL did back in the day.